New spray technology has been developed for spraying compositions with markedly reduced solvent emissions by using environmentally acceptable supercritical fluids or subcritical compressed fluids such as carbon dioxide as a substitute for the solvent fraction in solvent-borne compositions that is needed to obtain low spray viscosity. For coating compositions, solvent reductions up to 80 percent have been demonstrated, because only enough solvent for film coalescence and leveling is used.
Supercritical fluid applications and properties are reviewed by K. Johnston in "Supercritical Fluids", Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-Interscience, New York, 1984, and by M. A. McHugh and V. Krukonis in "Supercritical Fluid Extraction", Butterworths, Boston, 1986. An important property of supercritical fluids is that density, and hence solubility, can change markedly with small changes in pressure. Guckes et al. in U.S. Pat. No. 4,946,940 disclose a separation method in which methane is used as a phase separation agent to recover ethylene-propylene rubber from the hexane solvent reaction medium in which the solution polymerization process is carried out.
Although the supercritical fluid spray methods have been highly successful, one difficult problem that is created is that the reformulated composition, which is called a concentrate, has much higher viscosity after the dilution solvent is eliminated, typically 800 to 5000 centipoise or higher. Only when the concentrate is mixed with supercritical fluid is a low viscosity obtained. This makes manufacture, material handling and transfer, and other preparation operations, before the concentrate is sprayed, much more difficult than with conventional compositions that contain diluent solvents and have low viscosity, typically below 100 centipoise.
In addition to high viscosity, another difficult problem comes from concentrated reactive compositions, such as thermosetting systems or compositions with catalysts. The higher reactant concentration often significantly increases reactivity such that pot life becomes too short to spray the composition industrially.
Therefore, the ability to use additional solvent to manufacture, pump, meter, blend, mix, filter, and otherwise process concentrates at low viscosity like conventional compositions and to then separate the additional solvent just prior to spraying would be of great benefit.
There is therefore clearly a need to be able 1) to use excess diluent solvent for manufacturing, transporting, processing, and preparing compositions for spraying with supercritical fluids or subcritical compressed fluids, in order to avoid the problems created by viscous concentrates, and 2) to separate the excess diluent solvent just prior to spraying the composition, in order to minimize emissions of organic solvents from the sprayed composition.